Ultra-high pressure and ultra-reduced minerals in ophiolites may form by lightning strikes

Abstract

Since ultra-high pressure (UHP) minerals have been discovered in ophiolites from Tibet and the Polar Urals, it is speculated that the mantle sections of ophiolites may originate deep within the mantle. The UHP minerals are frequently found together with ultra-reduced silicides, carbides, and nitrides. Consequently, it is argued that the deep mantle, or at least domains within it, must be highly reduced, so reduced that practically all transition elements at depth are present in the metallic state. We find it problematic to rewrite the history of ophiolite complexes based on these observations and suggest we should search for alternative and more realistic modes of origin. Electric discharge experiments at >6000 K reported here show that the UHP and highly reduced phase assemblages may precipitate from plasmas. We argue that the mineral assemblages may originate by lightning strikes. As such, they may not record the origin and emplacement history of the mantle lithologies within which they occur.

Tsikourasite, Mo3Ni2P1+x (x < 0.25), a New Phosphide from the Chromitite of the Othrys Ophiolite, Greece

“…Natural phosphides have been reported in the mineralogical assemblages described in a few fulgurites [37] as well as being very common in meteorites. However, the probability of intercepting a fragment of a meteorite or a fulgurite in the Otrhys ophiolite during the sampling of the studied chromitite, seems very unlikely, because of the lack of high-pressure minerals in our assemblage, which would have formed during any impact [38]. Therefore, we argue that tsikourasite is natural and terrestrial in origin, and was crystallized in mantle derived rocks in a local reducing environment, perhaps during the serpentinization process at low temperature.…”

X-ray Laue Microdiffraction and Raman Spectroscopic Investigation of Natural Silicon and Moissanite

“…Many synthetic routes have been reported (see review by Abderrazak and Hmida [11]), with variations in the resulting crystal properties, but none truly mimic geological conditions, which range from relatively low pressure (upper crust) to high pressure (mid to lower mantle), high temperature, and very low oxygen fugacity. For more than a century, natural occurrences of moissanite have been reported in the literature and these reports show that SiC origin may be broken up into three categories: (i) high-, ultra-high pressure environments, such as upper-and lower mantle and mantle transition zone, and even core-mantle boundary; (ii) ambient/low-pressure environments, such as metamorphic, magmatic rocks and hydrothermal processes recorded in both continental and oceanic crust; or (iii) formation of SiC and native metals during lightning strikes in the ophiolitic rocks exposed on the Earth's surface [12][13][14][15][16][17][18][19][20]. All require temperature ranged from 700-800 • C to 2500 • C, and extremely low oxygen fugacity, e.g.~8 to <12 log-bar units [21], or 6 to 8 log-bar units [22] below the iron-wüstite (IW) buffer.…”

Factors Controlling the Chromium Isotope Compositions in Podiform Chromitites

“…Chromitites in ophiolite complexes are studied extensively, in particular with respect to mantle-lithospheric slab interactions, post magmatic processes and subduction recycling scenarios [1][2][3][4][5][6][7][8]. Chromitites differ in their major and trace element compositions, including their platinum-group element (PGE) content, and also have been shown to exhibit variations in their chromium isotope signatures.…”